Acetylene gas is widely used in many applications such as metal working operations. Generally acetylene gas is provided to the use point in cylinders which are filled at a central filling location and distributed to the use point. When the acetylene gas in the cylinder has been completely used up at the use point, the empty cylinder is returned to the central filling location for refilling.
As is well known, acetylene gas is very dangerous and unstable, and is highly susceptible to violent decomposition from an incidental energy input such as heat or mechanical impact. For this reason an acetylene gas cylinder has some important differences from a typical gas cylinder such as an oxygen cylinder. Whereas a typical gas cylinder is hollow inside, an acetylene gas cylinder is characterized by a filler material which fills the inside of the cylinder. The filler material is introduced into the cylinder as a slurry and heat treated to form solidified porous filler material throughout the interior cavity. Typically the filler material has a void space of about 90 percent of the cylinder interior. Often the filler material is a mixture of silica, lime and asbestos. Other suitable filler materials include a charcoal monolithic material made of charcoal, Portland cement and Kieselguhr.
When the acetylene gas cylinder is to be charged with acetylene it is first charged with a liquid solvent, such as acetone. Then the acetylene gas is charged into the cylinder under pressure and is dissolved in the solvent until it is needed for use. In this way, the acetylene gas is not allowed to build up as a gas within the cylinder and is thus much safer to transport and store.
It is important that the filler material retain its integrity throughout the life of the cylinder. A crack or large void in the filler material can allow a dangerous buildup of acetylene gas to occur. However, as can be appreciated it is quite difficult to determine the integrity of the filler material since it cannot be visually inspected. Tests to determine the integrity of acetylene cylinder filler material are generally expensive, time consuming or not very reliable. Thus it is desirable to provide an inexpensive test procedure to quickly and accurately determine the integrity of acetylene cylinder filler material.
A known method to detect cracks and other flaws in structures, such as pressure vessels, is acoustic emission testing which is based on the principle that a crack or other flaw in a structure will enlarge as a load is applied to the structure and that such enlargement will release energy partly in the form of stress waves which can be sensed and recorded. In the case of pressure vessels a typical acoustic emission test comprises filling the pressure vessel with a pressurizing fluid, such as a gas, and sensing the acoustic emission events which occur in the vessel walls by means of sensors attached to the exterior walls. An acoustic emission event is generated by a sharp release of energy partly resolved as stress waves, as the force holding material together is broken. By measuring such events one can determine the condition of the pressure vessel walls.
Acoustic emission testing has two general characteristics which are of importance herein. The first is an essentially exponential or non-linear generation of acoustic emission counts with increasing load. The second is termed the "Kaiser effect" which is the non-reversibility of acoustic emission testing. Once a structure has been stressed to a given load and that load released, a subsequent loading will not generate acoustic emission until the previous maximum load has been exceeded. That is, once a flaw is caused to grow under stress so as to generate acoustic emission and that stress removed, the flaw will not return to its pre-stress condition and a subsequent application of stress will not cause the flaw to grow with consequent acoustic emission until the subsequent stress exceeds the previously applied stress. Therefore when one tests a structure by measuring acoustic emission one expects to get a non-linear relationship of acoustic emission to increasing load due to (a) the inherent nature of materials to exponentially release energy, as their failure point is approached and (b) the Kaiser effect wherein a pre-stressed flaw will not generate acoustic emission until the prior maximum loading on that flaw is exceeded.
Acoustic emission testing is a very versatile non-destructive testing technique. Although it would be desirable to employ acoustic emission testing to determine the integrity of acetylene cylinder filler material, of course, sensors cannot be placed on the filler material. Since the sensors must be placed on the cylinder shell, it is not evident that an accurate reading of acoustic emission specific to the filler material can be obtained.
It is therefore an object of this invention to provide an uncomplicated and inexpensive non-destructive test to determine the integrity of cylinder filler material and the consequent suitability of the cylinder for use.
It is another object of the invention to employ acoustic emission testing to accurately determine the integrity of cylinder filler material.